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Effect of Double-Hyperconjugation on the Apparent Donor Ability of Σ-Bonds: Insights from the Relative Stability of Δ-Substituted Cyclohexyl Cations

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Effect of Double-Hyperconjugation on the Apparent Donor Ability of Σ-Bonds: Insights from the Relative Stability of Δ-Substituted Cyclohexyl Cations Effect of Double-Hyperconjugation on the Apparent Donor Ability of σ-Bonds: Insights from the Relative Stability of δ-Substituted Cyclohexyl Cations Igor V. Alabugin* and Mariappan Manoharan Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390 [email protected] Received September 27, 2004 A combination of electronic, structural, and energetic analyses shows that a somewhat larger intrinsic donor ability of the C-H bonds compared to that of C-C bonds can be overshadowed by cooperative hyperconjugative interactions with participation of remote substituents (double hyperconjugation or through-bond interaction). The importance of double hyperconjugation was investigated computationally using two independent criteria: (a) relative total energies and geometries of two conformers (“hyperconjomers”) of δ-substituted cyclohexyl cations (b) and natural bond orbital (NBO) analysis of electronic structure and orbital interactions in these molecules. Both criteria clearly show that the apparent donor ability of C-C bonds can vary over a wide range, and the relative order of donor ability of C-H and C-C bonds can be easily inverted depending on molecular connectivity and environment. In general, relative donor abilities of σ bonds can be changed by their through-bond communication with remote substituents and by greater polariz- ability of C-X bonds toward heavier elements. These computational results can be confirmed by experimental studies of conformational equilibrium of δ-substituted cyclohexyl cations. Introduction the relative ability of many common σ donors, including the most ubiquitous case of C-HvsC-C bonds, is still Hyperconjugation, or delocalization which involves σ widely debated.4 An often discussed example involves bonds, manifests itself in numerous stereoelectronic stereoelectronic control in nucleophilic addition to cyclo- 1,2 effects controlling organic structure and reactivity. In hexanones and related compounds. A widely discussed particular, such interactions can deliver electron density model proposed by Cieplak suggests that the decisive to electron-deficient centers and, thus, are often sug- stereoelectronic factor determining axial approach of gested to determine stereoselectivity of organic reactions nucleophilic attack on the CdO group involves electron where the transition states for stereodivergent pathways donation from axial C-H bonds to the antibonding orbital are characterized by different relative arrangements of associated with the incipient (forming) bond between donor and acceptor orbitals. incoming nucleophile (Nu) and the carbonyl carbon. In - - It is well established that donors such as C Si, C Ge, this model, the axial approach is favored because the - and C Sn bonds are capable of providing significant above stabilizing interaction is more efficient than the 3 stabilization to a developing positive charge. However, analogous interaction with C-C-bonds during the equa- torial approach of Nu (Figure 1).5 The larger donor ability (1) (a) Kirby, A. J. The Anomeric Effect and Related Stereoelectronic Effects at Oxygen; Springer-Verlag: Berlin, 1983. (b) The Anomeric Effect and Associated Stereoelectronic Effects; Thatcher, G. R. J., Ed.; (3) (a) Green, A. J.; Giordano, J.; White, J. M. Aust. J. Chem. 2000, ACS Symposium Series 539; American Chemical Society: Washington, 53, 285. For a review of interactions of C-MR3 bonds with acceptor DC, 1993. (c) Juaristi, E.; Guevas, G. The Anomeric Effect; CRC orbitals, with remote electron-deficient orbitals, π systems, etc., see: Press: Boca Raton, FL, 1994. (d) Juaristi, E., Ed. Conformational White, J. M.; Clark, C. I. Top. Stereochem. 1999, 22, 137. (b) Lambert, Behavior of Six-Membered Rings; VCH Publishers: New York, 1995. J. B.; Zhao, Y.; Emblidge, R. W.; Salvador, L. A.; Liu, X.; So, J.-H.; (e) For a recent experimental example and leading references, see Chelius, E. C. Acc. Chem. Res. 1999, 32, 183. Lambert, J. B.; Wang, also: Uehara, F.; Sato, M.; Kaneko, C.; Kurihara, H. J. Org. Chem. G. T.; Teramura, D. H. J. Org. Chem. 1988, 53, 5422. Lambert, J. B.; 1999, 64, 1436. Wang, G. T.; Finzel, R. B.; Teramura, D. H. J. Am. Chem. Soc. 1987, (2) Cohen, T.; Lin, M.-T. J. Am. Chem. Soc. 1984, 106, 1130. 109, 7838. Lambert, J. B. Tetrahedron 1990, 46, 2677. (c) Cook, Mi. Danishefsky, S. J.; Langer, M. Org. Chem. 1985, 50, 3672. Rychnovsky, A.; Eaborn, C.; Walton, D. R. M. J. Organomet. Chem. 1970, 24, 293. S. D.; Mickus D. E. Tetrahedron Lett. 1989, 30, 3011. Vedejs, E.; Dent, (4) For the most recent experimental study and an excellent historic W. H. J. Am. Chem. Soc. 1989, 111, 6861. Juaristi, E.; Cuevas, G.; survey of this problem, see: Spiniello, M.; White, J. M. Org. Biomol. Vela, A. J. Am. Chem. Soc. 1994, 116, 5796. Salzner, U.; Schleyer P. Chem. 2003, 1, 3094. v. R. J. Org. Chem. 1994, 59, 2138. Vedejs, E.; Dent, W. H.; Kendall, (5) (a) Cieplak, A. S. J. Am. Chem. Soc. 1981, 103, 4540. (b) Cieplak, J. T.; Oliver, P. A. J. Am. Chem. Soc. 1996, 118, 3556. Cuevas, G.; A. S.; Tait, B. D.; Johnson, C. R. J. Am. Chem. Soc, 1989, 111, 8447. Juaristi, E.; Vela, A. THEOCHEM 1997, 418, 231. Anderson, J. E.; For the Felkin-Ahn model of stereoselectivity of these reactions which Cai, J.; Davies, A. G. J. Chem. Soc., Perkin Trans. 2 1997, 2633. depends on acceptor ability of σ bonds, see: Cherest, M.; Felkin, H.; Wiberg, K. B.; Hammer, J. D.; Castejon, H.; Bailey, W. F.; DeLeon, E. Prudent, N. Tetrahedron Lett. 1968, 2199. Cherest, M.; Felkin, H. L.; Jarret, R. M. J. Org. Chem. 1999, 64, 2085. Juaristi, E.; Rosquete- Tetrahedron Lett. 1968, 2205. Cherest, M. Tetrahedron 1980, 36, 1593. Pina, G. A.; Vazquez-Hernandez, M.; Mota, A. J. Pure Appl. Chem. Ahn, N. T.; Eisenstein, O. Tetrahedron Lett. 1976, 155. Ahn, N. T. Top. 2003, 75, 589. Curr. Chem. 1980, 88, 145. 10.1021/jo048287w CCC: $27.50 © 2004 American Chemical Society Published on Web 11/12/2004 J. Org. Chem. 2004, 69, 9011-9024 9011 Alabugin and Manoharan FIGURE 2. Axial and equatorial “hyperconjomers” of cyclo- hexyl cations. greater donating ability of C-H bonds compared to that of C-C bonds. Although these results are consistent with the trends in 13C NMR chemical shifts of the â-carbon in â-substituted styrenes in solvents of different polarity,13 FIGURE 1. Most important transition-state-stabilizing hy- the opposite trend found in the gas-phase pyrolysis of perconjugative interactions for axial and equatorial nucleo- 1-arylethyl acetates cast a shadow of doubt at the original philic addition to cyclohexanone according to Cieplak’s model. interpretation of the Nathan-Baker effect.14 To compli- of C-H bonds compared to that of σ C-C bonds is the cate the matter even further, a recent study of two cornerstone of this model.6 This suggestion stimulated conformers of cyclohexyl cations with two distinctly many experimental7 and theoretical8 studies, some of different patterns of hyperconjugative interactions (“hy- which questioned the order of relative donor abilities of perconjomers”, Figure 2) found that a switch in the order - - σ C-C and C-H bonds. of the apparent donor ability of C H and C C bonds in The uncertainly of relative donor abilities of C-H and solution vs the gas phase occurs in the opposite direc- 15 C-C bonds is not surprising because the difference tion. Although the isomer stabilized preferentially by - f + between these two bonds is not large in ground-state σ(C H) n(C ) hyperconjugation is more stable accord- neutral molecules. For example, a careful low-tempera- ing to the gas-phase computations, the opposite order of ture X-ray structural study by Spinello and White found stability was observed in experimental studies in solution that the differences in donor ability of C-C and C-H and in computations including solvation effects (vide bonds toward σ(C-O) acceptors of variable electronic infra). demand are comparable to the experimental uncertainty Taking into account the fundamental importance of of measurements.4 Two recent computational studies also this question and conflicting literature results, we de- found that the differences are small. Natural bond orbital cided to determine the intrinsic order of donor ability of - - (NBO) analysis indicates that C-H bonds are slightly C H and C C σ bonds in systems where the differences better donors than σ C-C bonds in cyclohexane and in donor ability of σ bonds may be significantly large to related molecules.9 A similar conclusion was made by lead to experimentally observable consequences. Our Rablen and co-workers in a theoretical study on the origin choice of the model systems was inspired by the above 15 of gauche effects in substituted fluoroethanes.10 Note, elegant recent study of Rauk and co-workers who however, that the small differences in neutral molecules reported the dramatic effect of hyperconjugation on two 16 do not prevent these effects from becoming significant isomeric chair conformers of 1-Me-1-cyclohexyl cation in species with increased electron demand.11 differing in having the carbocation p-orbital oriented This importance has been well-recognized but also either “axially” or “equatorially”. These conformers have debated for quite a long time. For example, Nathan and distinctly different modes of hyperconjugative stabiliza- 15 Baker reported that a Me group provides more stabiliza- tion. The axial cationic orbital in the first hypercon- - tion to the developing positive charge at the p-benzylic jomer interacts strongly with the adjacent axial C H position than Et, i-Pr, and t-Bu groups in solvolysis of bonds whereas the equatorial vacant p-orbital in the p-alkyl-substituted benzyl bromides (Me > Et > i-Pr > second cation interacts most strongly with the anti- - t-Bu)12 and attributed this order of reactivity to the periplanar C C bonds (Figure 2).
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